The period from May to August was the driest four months the United States has experienced since 1895, according to an analysis released April 11 led by the U.S. National Oceanic and Atmospheric Administration (NOAA).

A drought as severe as last year's happens only once every couple of hundred years, said Martin Hoerling, an NOAA meteorologist who led the drought assessment. "It was a pretty rare event," he added during a teleconference with reporters. (Related pictures: "Surprising Effects of the U.S. Drought.")

The researchers were careful to point out that the 2012 drought was not a continuation of the 2011 drought that left the southern Plains area, including Texas and New Mexico, high and dry.

But Hoerling and his colleagues found that natural swings in wind patterns and humidity over the Great Plains and the Gulf of Mexico were the major culprits in the 2012 drought.

An Unexpected Combination

A drop in the strength of the wind coming up from the Gulf of Mexico—which usually brings moisture into the Great Plains in summer—combined with unusually low humidity over the area to produce conditions drier than those of the 1930's drought.

A ridge of high pressure squatting over the northern Plains kept cold fronts in Canada from coming down into the central Plains, further stacking the deck in the drought's favor, according to the NOAA report.

Although researchers can get a sense of whether a drought will hit later in the year by looking at the most recent winter precipitation patterns, they had no warning about the 2012 drought.

In the decades leading to 2012, summers in the Great Plains were actually cool and wet, explained Hoerling. When looked at in terms of the historical record, this severe drought was a surprise, he said.

And does global warming get any of the blame? Hoerling and his colleagues did not discount the possibility, but more study will be needed to see if researchers can pick out a climate change signal from the noise of natural atmospheric swings.

"You're looking for a tree within a forest of natural variability," Seager said.